I heard a loud BANG in the garage and have not been able to open the garage door since then. What happened?

It is likely that you have a broken garage door spring that should be replaced by a licensed garage door service technician. If your garage door springs are on a shaft across the top of the garage door, you will notice that one of the garage door springs has broken into two separate pieces.
 Even though only one garage door spring is broken, you should have a licensed garage door technician replace both of your garage door springs because the other spring has been fatigued and will not be easily balanced with a new garage door spring.